UNI-T UT366A Pressure Reading Unstable – Troubleshooting Guide Malaysia

An unstable pressure reading on the UNI-T UT366A Digital Manometer does not always mean the instrument is faulty. Fluctuating values can be caused by changing airflow, leaking hoses, incorrect connections or unsuitable measurement points.

Before sending the instrument for checking, inspect the complete measurement setup and repeat the test under stable operating conditions.

Common Symptoms

Users may notice:

Pressure readings continuously moving up and down
The display does not return to zero
P1 and P2 readings appear inconsistent
Positive pressure changes to negative pressure
Differential-pressure readings are much higher than expected
The reading changes when the hose is moved
Air-velocity readings fluctuate when using a Pitot tube
The same system produces different results during repeated tests

The symptom and application should be recorded before troubleshooting begins.

Check the Pressure Hoses

The pressure hose is one of the most common causes of unstable or incorrect readings.

Inspect the hose for:

Loose connections
Small cracks or punctures
Hardened material
Folded or kinked sections
Internal blockage
Moisture or dirt inside the hose
Incorrect hose diameter
Excessive hose movement

Reconnect the hose securely and repeat the measurement. If the value changes when the hose is touched or moved, leakage or a poor connection may be present.

Confirm the P1 and P2 Connections

For differential-pressure measurement, the two pressure points must be connected correctly.

If P1 and P2 are reversed:

The pressure direction may appear opposite
A positive value may become negative
The LED pressure indicator may show an unexpected colour
The reading may be misunderstood even though the instrument is functioning normally

Mark the hoses or use different colours to prevent connection mistakes during repeated HVAC inspections.

Check Whether the System Is Stable

The UT366A displays actual pressure changes. Therefore, a changing reading may reflect a changing system rather than an instrument problem.

Possible system-related causes include:

Variable fan speed
A variable air volume control operating
A damper opening or closing
Doors or access panels being opened
Intermittent exhaust operation
Pressure changes between connected rooms
Pulsating airflow from machinery
A clogged filter moving under airflow
Outdoor wind affecting the ventilation system

Confirm that the AHU, fan or ventilation system is operating steadily before comparing results.

Inspect the Pressure Test Point

A poor test location can create unstable readings.

Avoid measuring:

Immediately after a sharp duct bend
Directly beside a fan
Close to a damper
In highly turbulent airflow
At a blocked or damaged pressure tap
At a point affected by direct air impact when static pressure is required

Use suitable pressure points and allow the display to stabilise before recording the value.

Check the Zero Reading

Disconnect both pressure hoses and leave the ports open under the same ambient pressure.

Observe whether the display settles at or near zero. If the value remains offset:

Check whether pressure is trapped inside a hose
Make sure both ports are open
Allow the instrument to adjust to the surrounding temperature
Restart the instrument according to the operating procedure
Repeat the zero check in a stable location

A persistent zero offset may require further inspection or calibration verification.

Check the Selected Measurement Unit

The UT366A supports several pressure units, including:

kPa
mbar
bar
psi
mmHg
inH₂O
inHg
mmH₂O
ftH₂O
hPa

A reading may appear incorrect when the wrong unit is selected. Always compare the value using the same unit stated in the AHU, filter or ventilation specification.

Its unit-memory function may retain the previously selected unit, so technicians should confirm the display before every inspection.

Troubleshooting Pitot Tube Measurements

When using the optional Pitot tube, unstable air-velocity readings may be caused by:

Incorrect Pitot tube direction
The tube not facing directly into the airflow
Turbulence inside the duct
Testing too close to a bend or damper
Loose black or white silicone hoses
Reversed pressure connections
Blocked Pitot tube openings
Moving the tube while recording the value
Measuring at only one point in an uneven airflow profile

Hold the Pitot tube steadily, confirm its orientation and take readings at several positions across the duct.

Check the Battery Condition

A low battery can affect normal instrument operation.

The UT366A includes a low-battery indicator and uses three AAA batteries. Replace all batteries with a matching set when the indicator appears or when instrument behaviour becomes inconsistent.

After replacing the batteries, restart the instrument and repeat the zero and pressure checks.

Compare with a Known Pressure Source

If possible, compare the UT366A against:

A known pressure source
Another verified manometer
A controlled test setup
A recent calibration reference

Do not use an unstable ventilation system as the only basis for deciding whether the instrument is accurate.

A controlled comparison helps separate an instrument issue from a hose, connection or system problem.

Checking, Calibration or Repair?

These terms describe different actions.

Checking determines whether the instrument is operating normally and whether the accessories or setup are causing the problem.

Calibration compares the UT366A against a traceable pressure reference and records any measurement deviation.

Repair may be required when the instrument has a damaged pressure port, internal sensor problem, display fault, battery-contact issue or physical damage.

Calibration does not automatically repair a defective instrument.

When to Request Professional Inspection

Arrange further checking when:

The display does not return near zero
Readings remain unstable with both ports open
A pressure port is damaged
The unit was dropped
Moisture entered the pressure ports
The instrument differs significantly from a verified reference
The problem remains after replacing hoses and batteries
Repeat measurements are inconsistent under controlled conditions

Do not apply pressure beyond the instrument’s specified range, as excessive pressure may damage the internal sensor.

Contact MTM Precision

For initial assessment, WhatsApp MTM Precision with:

A clear photo of the UT366A
A photo of the model label
A short video showing the unstable reading
The selected pressure unit
Photos of the hose connections
A description of the equipment being tested

This information helps determine whether the problem is related to the setup, accessories or instrument.

MTM Precision Sdn Bhd

Showroom & Service Centre:
No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia

Tel: 03-8080 7172
WhatsApp: +6016-660 7346
Email: mtmpre@yahoo.com
Website: www.mtmpre.com.my